We usually first become aware of the fact that we are hungry when we
feel
"hunger pangs," which are just our stomach contractions.
For
many people, this is a strong incentive to eat, but it is not,
physiologically, the most significant indication of hunger.

More important is the level of glucose (blood sugar) in the
blood. Most of the food you eat gets converted to glucose, much
of which is converted by the liver into fat for later use. When
the levels of glucose are low, the liver sends signals to the
hypothalamus - specifically, the lateral hypothalamus - that
levels are low. The hypothalamus in turn triggers
whatever habits you have accumulated relating to food seeking and
consumption.

Another portion of the hypothalamus (the paraventricular
hypothalamus) actually tells you more specifically which foods you
need, and seems to be
responsible for many of our "cravings."

The feeling that it is time to stop eating is called satiety.
Again, the first indicators may be the distension of the stomach and
the intestines
-- that full or even bloated feeling we all know from Thanksgiving
dinner.

There is also a certain hormone called CCK (Cholecystokinin) that is
released when food begins
to move
from the stomach to the intestines and that signals the hypothalamus
(this
time,
the ventromedial hypothalamus) that it's time to stop eating.

And there is a hormone released by the fat cells themselves called leptin
that decreases appetite via the hypothalamus.

I'm sure you've all talked about one person having a better metabolism
than another. Some people just seem to burn calories as quick as
they
eat them, while others gain weight just by looking at food. This
is
called the set point hypothesis. It suggests that
everyone has
a certain metabolic set point, a certain weight that your body is
geared
towards, which is determined by your metabolism, or the rate at which
you
burn calories. Different people have different set points, and it
is
believed that these set points can change depending on a number of
factors,
including eating patterns and exercise.

The Psychology of Hunger

Hunger is not, of course, entirely a physical process. For one
thing,
the cultural and even individually learned preferences and eating
habits
can make a difference. For example, some of us eat regular meals
and
rarely snack, while others just nibble throughout the day. Every
culture
has its collection of foods that are preferred and those that are
avoided.
Many people like the burned flesh of large herbivores (i.e. a steak);
others
prefer raw squid; others still prefer to graze on a variety of
vegetation....

Our culture and upbringing also provide us with various beliefs and
attitudes
about food and eating in general, and our personal memories can
influence
our eating behaviors as well. Some of us grow up with the
idea
that we should never waste food, for example, and many of us have
particular
attachments to what are sometimes called "comfort foods."

Eating is a social thing in human beings and can give one a sense of
love
and belonging. It has been suggested that for some people, food
is
a "substitute" for the love they crave. Also, some foods -
chocolate and ice cream come to mind - seem to reduce anxiety and
stress for many of
us.

One of the strongest learning experiences both humans and animals
have is called taste aversion: If we get sick soon after
eating something,
we can develop a instant dislike for that food for the rest of our
lives!
Children often say they are "allergic" to one food or another when this
happens.

Eating Disorders

As is the case with anything as important as eating, human beings
have developed a number of eating disorders. One is called bulimia
nervosa,
and consists of a pattern of "binging" and "purging" - periods of
sometimes
extreme overeating followed by periods of vomiting or the use of
laxatives.

Bulimics are usually obsessed with maintaining or reducing their
weight. They tend to suffer from depression, anxiety, poor
self-esteem, and poor impulse
control. They tend to come from families with a history of
emotional
problems such as depression, as well as families with obesity problems.

Anorexia nervosa is another eating disorder which involves
dieting to the point of starvation. The "rule of thumb" is that
you are seriously underweight if you are more than 15% below your ideal
weight. Anorexics often use vomiting and laxatives, just like the
bulimics. They have an intense fear of being fat and are obsessed
with being thin. They often have a distorted body image, meaning
that when they look in the mirror, they tend to see someone overweight,
when others see them as walking skeletons. Anorexics often come
from very competitive, demanding families, and are often
perfectionists with a strong need to control all aspects of their
lives.

Physiologically, anorexia has been linked to abnormal levels of the
neurotransmitter
seratonin, which is involved in eating regulation. Twin research
suggests
that there may be a genetic aspect to anorexia as well.

Most anorexics and bulimics are young women, including from 1 to 4%
of
high school and college girls. It may be that there are
physiological aspects of female adolescence contributing to the
problem, but we should note
that 10% of teenages with anorexia or bulimia are boys. But a
significant root of these disorders is likely social: In our
society at this point in history, the
standards
of beauty tend to emphasize thinness, and women in particular tend to
be
judged on the basis of beauty, sometimes to the exclusion of all
else.
Certainly, if you look at many magazines for young women, or
advertisement
directed at them, you would think that looks are everything, and that
fat
is the kiss of death for self-worth!

It is interesting to note that, whereas the average American woman
is 5
foot 4 inches and weighs 142 pounds, the average model is 5 foot 9
inches and weights 110 pounds. If Barbie, that childhood ideal of
feminine beauty, were full size, her figure would read 36-18-33!

It is interesting that cultures with standards of beauty that have
more respect for a woman's personality or other traits, and cultures
that appreciate heavier women, have little or no trouble with bulimia
or
anorexia.

Obesity

For all the suffering that bulimia and anorexia are responsible for,
another
eating disorder causes far more: Obesity. The "rule
of
thumb" is that you are obese if you are more than 35% over your ideal
weight.
By that standard, about 21% of Americans are obese. Europeans and
others
with "slimmer" populations shouldn't gloat too much, however:
This
tendency is actually world-wide!

Physiologically, obesity is strongly associated with diseases such
as diabetes, high blood pressure, heart disease, and some
cancers.
Psychologically, the toll is great as well, and obesity is associated
with
depression. Even sociologically, obese people face considerable
discrimination,
from childhood teasing to denial of employment in adulthood. And
unlike
other kinds of discrimination, people actually consider obesity the fat
person's
own fault!

Genetics is a major cause of obesity, and somewhere between 40 and
70 percent
of the variation in body mass seems to be genetic. Our ancestors
that passed those genes down to us didn't get fat, mainly because they
didn't have as much easily available food as we do, and because they
had to work harder and walk further to get by.

But learning
is
also a major factor, including childhood eating patterns and a
sedentary lifestyle.
Our culture doesn't help at all, in that our food and snack industry
spends
billions of dollars every year encouraging us (including children) to
eat
food filled with sugar and fat. Often the same companies then
make
billions (33 billion dollars in 1996) selling us weight loss programs
and
products!

Most people attempt to deal with obesity with dieting. In fact 80%
of
all American women diet, and 25% of men. 50% of girls below the
age
of 18 have dieted! Unfortunately, although dieting often works in
the short-run and for small amounts, it usually fails in the long-run
for the people who are obese.

Dieting is made even more difficult by the way in which "set point"
works:
When you diet, your body thinks you are starving, and so readjusts your
metabolism
to be more efficient, thereby causing you to need less food to maintain
your
body, and making it even more difficult to lose weight. Although you do
need
to eat more than you burn up to get fat, once you are
fat,
you don't have to eat much at all!

Diet and exercise are, of course, the only hope, but the failure
rate is
so great - over 95% - that doctors usually focus on treating the
diseases
that
result from obesity rather than dealing with the obesity itself.
One
might want to keep in mind that weight training, which increases muscle
mass,
helps: Muscles use up more calories even in a resting state than
other
tissues. There is also some hope for the future in medical
research,
including research on the effects of leptin and possible genetic
interventions.